Complement regulation in human atherosclerotic coronary lesions

The Complement System and C4b-Binding Protein: A Focus on the Promise of C4BPα as a Biomarker to Predict Clopidogrel Resistance

The complement system plays a dual role in the body, either as a first-line defense barrier when balanced between activation and inhibition or as a potential driver of complement-associated injury or diseases when unbalanced or over-activated. C4b-binding protein (C4BP) was the first circulating complement regulatory protein identified and it functions as an important complement inhibitor. C4BP can suppress the over-activation of complement components and prevent the complement system from attacking the host cells through the binding of complement cleavage products C4b and C3b, working in concert as a cofactor for factor I in the degradation of C4b and C3b, and consequently preventing or reducing the assembly of C3 convertase and C5 convertase, respectively. C4BP, particularly C4BP α-chain (C4BPα), exerts its unique inhibitory effects on complement activation and opsonization, systemic inflammation, and platelet activation and aggregation. It has long been acknowledged that crosstalk or interplay exists between the complement system and platelets. Our unpublished preliminary data suggest that circulating C4BPα exerts its antiplatelet effects through inhibition of both complement activity levels and complement-induced platelet reactivity. Plasma C4BPα levels appear to be significantly higher in patients sensitive to, rather than resistant to, clopidogrel, and we suggest that a plasma C4BPα measurement could be used to predict clopidogrel resistance in the clinical settings.

Cell-autonomous regulation of complement C3 by factor H limits macrophage efferocytosis and exacerbates atherosclerosis

Complement factor H (CFH) negatively regulates consumption of complement component 3 (C3), thereby restricting complement activation. Genetic variants in CFH predispose to chronic inflammatory disease. Here, we examined the impact of CFH on atherosclerosis development. In a mouse model of atherosclerosis, CFH deficiency limited plaque necrosis in a C3-dependent manner. Deletion of CFH in monocyte-derived inflammatory macrophages propagated uncontrolled cell-autonomous C3 consumption without downstream C5 activation and heightened efferocytotic capacity. Among leukocytes, Cfh expression was restricted to monocytes and macrophages, increased during inflammation, and coincided with the accumulation of intracellular C3. Macrophage-derived CFH was sufficient to dampen resolution of inflammation, and hematopoietic deletion of CFH in atherosclerosis-prone mice promoted lesional efferocytosis and reduced plaque size. Furthermore, we identified monocyte-derived inflammatory macrophages expressing C3 and CFH in human atherosclerotic plaques. Our findings reveal a regulatory axis wherein CFH controls intracellular C3 levels of macrophages in a cell-autonomous manner, evidencing the importance of on-site complement regulation in the pathogenesis of inflammatory diseases.

Therapeutic Intervention with Anti-Complement Component 5 Antibody Does Not Reduce NASH but Does Attenuate Atherosclerosis and MIF Concentrations in Ldlr-/-.Leiden Mice

Background: Chronic inflammation is an important driver in the progression of non-alcoholic steatohepatitis (NASH) and atherosclerosis. The complement system, one of the first lines of defense in innate immunity, has been implicated in both diseases. However, the potential therapeutic value of complement inhibition in the ongoing disease remains unclear. Methods: After 20 weeks of high-fat diet (HFD) feeding, obese Ldlr-/-.Leiden mice were treated twice a week with an established anti-C5 antibody (BB5.1) or vehicle control. A separate group of mice was kept on a chow diet as a healthy reference. After 12 weeks of treatment, NASH was analyzed histopathologically, and genome-wide hepatic gene expression was analyzed by next-generation sequencing and pathway analysis. Atherosclerotic lesion area and severity were quantified histopathologically in the aortic roots. Results: Anti-C5 treatment considerably reduced complement system activity in plasma and MAC deposition in the liver but did not affect NASH. Anti-C5 did, however, reduce the development of atherosclerosis, limiting the total lesion size and severity independently of an effect on plasma cholesterol but with reductions in oxidized LDL (oxLDL) and macrophage migration inhibitory factor (MIF). Conclusion: We show, for the first time, that treatment with an anti-C5 antibody in advanced stages of NASH is not sufficient to reduce the disease, while therapeutic intervention against established atherosclerosis is beneficial to limit further progression.

Indoor-related microbe damage induces complement system activation in building users

In this comparative study, serum complement system antimicrobial activity was measured from 159 serum samples, taken from individuals from microbe-damaged (70 samples) and from reference buildings (89 samples). Antimicrobial activity was assessed using a probe-based bacterial Escherichia coli-lux bioluminescence system and comparison was made at a group level between the experimental and reference group. The complement activity was higher in users of microbe-damaged buildings compared with the reference group and the significant (P < 0.001) increase in activity was found in the classical reaction pathway. This study strengthens our notion that exposure to indoor-related microbe damage increases the risk for systemic subclinical inflammation and creates a health risk for building users.

The CD40-CD40L Dyad as Immunotherapeutic Target in Cardiovascular Disease

Chronic inflammation drives the development of atherosclerosis. Despite optimal treatment of classical cardiovascular risk factors, a substantial portion of the population has elevated inflammatory biomarkers and develops atherosclerosis-related complications, indicating that a residual inflammatory risk drives atherosclerotic cardiovascular disease in these patients. Additional anti-inflammatory therapeutic strategies are therefore required. The co-stimulatory molecule CD40 and its ligand CD40L (CD154) have a central role in the regulation of the inflammatory response during the development of atherosclerosis by modulating the interaction between immune cells and between immune cells and non-immune cells. In this review, we discuss the role of the CD40-CD40L dyad in atherosclerosis, and we discuss recent studies on the therapeutic potential of novel CD40-CD40L targeting strategies in cardiovascular medicine.

Serum complement C1q level is associated with acute coronary syndrome

BACKGROUND AND OBJECTIVES

The complement system plays an important role in the development of acute coronary syndrome (ACS). Complement C1q is an important initial component of the classical complement pathway and closely related to many chronic inflammatory diseases, including atherosclerosis (AS). We aimed to determine whether there was association between serum complement C1q and the severity of coronary stenosis.

SUBJECTS AND METHODS

320 patients who underwent coronary arteriography (CAG) were stratified into non-ACS group (control group, n = 74), unstable angina group (UA group, n = 197) and acute myocardial infarction group (AMI group, n = 49) according to the severity of coronary stenosis and clinical manifestations. The severity of coronary stenosis was represented in Gensini score, and serum complement C1q level was compared using immunity transmission turbidity among three groups.

RESULTS

The level of complement C1q in AMI group was lower significantly than control group and UA group (P < 0.05), but there was no correlation between serum complement C1q and Gensini score (β=-0.086, P = 0.125). In nitrate-taking patients, serum complement C1q had a negative association with Gensini score (r=-0.275, P = 0.001), and in non-smokers, there was also a negative correlation (β=-0.159, P = 0.036). After calibrating smoking, drinking or statins, the serum complement C1q levels of control group, UA group and AMI group decreased in sequence (P <  0.05). Logistic regression analysis showed that the decreasing of serum complement C1q was an unfavorable factor for acute myocardial infarction (OR=0.984, 95 %CI=0.972∼0.997, P = 0.015) and for ACS (OR=0.984, 95 %CI=0.971∼0.984, P = 0.025) in drinking patients. Regrettably, ROC curve suggested that the accuracy in diagnosing coronary atherosclerotic heart disease by serum complement C1q was low (AUC=0.568, 95 %CI= 0.492-0.644, P = 0.076, sensitivity 73.6 %, specificity 58.1 %).

CONCLUSION

Serum complement C1q in ACS patients, in particular AMI patients, showed lower level. This finding suggests further decrease of complement C1q level in ACS patients may be a contributory factor to instability or rupture of atherosclerotic plaques. Combined with other clinical indicators, it can be helpful to predict the risk and severity of coronary stenosis.

Complement Complex C5b-9 Levels Are Associated with the Clinical Outcomes of Acute Ischemic Stroke and Carotid Plaque Stability

The terminal complement complex C5b-9 plays an important role in acute ischemic stroke (AIS) and carotid atherosclerosis. However, the associations between serum C5b-9, the severity and outcome of AIS, and the stability of carotid plaques have not been well investigated. In this clinical study, 70 patients with AIS and 70 healthy controls were enrolled. Serum C5b-9 levels at 72 h after stroke onset were measured by enzyme-linked immunosorbent assay (ELISA). Infarct size, the National Institutes of Health Stroke Scale (NIHSS), the 90-day modified Rankin Scale (mRS), and carotid plaque and stenosis were evaluated. Serum C5b-9 levels were significantly higher in AIS patients than in healthy controls (p < 0.001) and were correlated with infarction sizes (p = 0.045) and the NIHSS (P = 0.035). Furthermore, 90-day mRS analysis demonstrated that the patients with poor outcomes had higher serum C5b-9 levels than those with good outcomes (P < 0.001). Moreover, serum C5b-9 levels in AIS patients with unstable carotid plaques were much higher than in those with stable carotid plaques (P = 0.009). Multivariate logistic regression indicated that C5b-9 could be an independent risk factor for AIS (P < 0.001) and unstable carotid plaques (P = 0.015). Therefore, complement complex C5b-9 may be a potential biomarker in predicting the severity and outcome, as well as the stability of carotid plaques, in AIS patients.

Impaired kidney function is associated with intraplaque hemorrhage in patients undergoing carotid endarterectomy

BACKGROUND AND AIMS

Previously, we showed that patients undergoing carotid endarterectomy have an increased risk for major atherosclerotic events in the presence of moderate or poor kidney function. Acceleration of vascular inflammatory responses is considered to be causally involved in progression of atherogenesis and poor outcome in chronic kidney disease patients. The association between kidney function and plaque composition has not been thoroughly investigated yet. The aim of this study was to investigate the association between kidney function and atherosclerotic plaque composition in patients undergoing carotid endarterectomy.

METHODS

Atherosclerotic plaques, harvested from 1796 patients who underwent carotid endarterectomy, were immunohistochemically stained for macrophages, smooth muscle cells, calcifications, collagen, microvessels, lipid core size and intraplaque hemorrhage. Cytokines were measured in plaque and plasma and associated with kidney function. Quantitative proteomics were performed on 40 carotid plaques and associated with kidney function.

RESULTS

Decreased kidney function was associated with increased odds ratio of intraplaque hemorrhage, OR 1.15 (95% CI; 1.02-1.29 (p = 0.024)) and increased odds ratio of fibrous-atheromatous plaques (plaques with lipid core presenting more than 10% of total plaque surface) OR 1.21 (95% CI; 1.07-1.38 (p = 0.003)) per decrease of 20 points in eGFR. Proteomics revealed that decreased kidney function was associated with upregulation of the classical pathway of the complement system and the intrinsic pathway of the coagulation system.

CONCLUSIONS

Decreased kidney function was associated with plaque hemorrhage but not with inflammatory plaque characteristics. Our data suggests that other pathways than the inflammation-pathway are involved in plaque vulnerability and poor outcome in patients with decreased kidney function.

Readapting the adaptive immune response - therapeutic strategies for atherosclerosis

Cardiovascular diseases remain a major global health issue, with the development of atherosclerosis as a major underlying cause. Our treatment of cardiovascular disease has improved greatly over the past three decades, but much remains to be done reduce disease burden. Current priorities include reducing atherosclerosis advancement to clinically significant stages and preventing plaque rupture or erosion. Inflammation and involvement of the adaptive immune system influences all these aspects and therefore is one focus for future therapeutic development. The atherosclerotic vascular wall is now recognized to be invaded from both sides (arterial lumen and adventitia), for better or worse, by the adaptive immune system. Atherosclerosis is also affected at several stages by adaptive immune responses, overall providing many opportunities to target these responses and to reduce disease progression. Protective influences that may be defective in diseased individuals include humoral responses to modified LDL and regulatory T cell responses. There are many strategies in development to boost these pathways in humans, including vaccine-based therapies. The effects of various existing adaptive immune targeting therapies, such as blocking critical co-stimulatory pathways or B cell depletion, on cardiovascular disease are beginning to emerge with important consequences for both autoimmune disease patients and the potential for wider use of such therapies. Entering the translation phase for adaptive immune targeting therapies is an exciting and promising prospect.

LINKED ARTICLES

This article is part of a themed section on Targeting Inflammation to Reduce Cardiovascular Disease Risk. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.22/issuetoc and http://onlinelibrary.wiley.com/doi/10.1111/bcp.v82.4/issuetoc.

Complement-mediated 'bystander' damage initiates host NLRP3 inflammasome activation

Complement activation has long been associated with inflammation, primarily due to the elaboration of the complement anaphylotoxins C5a and C3a. In this work, we demonstrate that the phagocytosis of complement-opsonized particles promotes host inflammatory responses by a new mechanism that depends on the terminal complement components (C5b-C9). We demonstrate that during the phagocytosis of complement-opsonized particles, the membrane attack complex (MAC) of complement can be transferred from the activating particle to the macrophage plasma membrane by a 'bystander' mechanism. This MAC-mediated bystander damage initiates NLRP3 inflammasome activation, resulting in caspase-1 activation and IL-1β and IL-18 secretion. Inflammasome activation is not induced when macrophages phagocytize unopsonized particles or particles opsonized with serum deficient in one of the terminal complement components. The secretion of IL-1β and IL-18 by macrophages depends on NLRP3, ASC (also known as PYCARD) and caspase-1, as macrophages deficient in any one of these components fail to secrete these cytokines following phagocytosis. The phagocytosis of complement-opsonized particles increases leukocyte recruitment and promotes T helper 17 cell (TH17) biasing. These findings reveal a new mechanism by which complement promotes inflammation and regulates innate and adaptive immunity.

Platelets and the complement cascade in atherosclerosis

Atherosclerosis and its late sequels are still the number one cause of death in western societies. Platelets are a driving force not only during the genesis of atherosclerosis, but especially in its late stages, as evidenced by complications such as arterial thrombosis, myocardial infarction, and ischemic stroke. Atherosclerosis is increasingly recognized as an inflammatory disease, influenced by various immune mechanisms. The complement system is part of our innate immune system, and its diverse roles in atherosclerosis have become evident over the past years. In this review we identify points of intersection between platelets and the complement system and discuss their relevance for atherosclerosis. Specifically, we will focus on roles for platelets in the onset as well as progression of the disease, a possible dual role for complement in the genesis and development of atherosclerosis, and review emerging literature revealing previously unrecognized cross-talk between platelets and the complement system and discuss its possible impact for atherosclerosis. Finally, we identify limitations of current research approaches and discuss perspectives of complement modulation in the control of the disease.

Role of vascular inflammation in coronary artery disease: potential of anti-inflammatory drugs in the prevention of atherothrombosis. Inflammation and anti-inflammatory drugs in coronary artery disease

Coronary artery disease (CAD) and acute myocardial infarction (AMI) are inflammatory pathologies, involving interleukins (ILs), such as IL-1β, IL-6 and tumor necrosis factor (TNF)-α, and acute phase proteins production, such as for C reactive protein (CRP). The process begins with retention of low-density lipoprotein (LDL) and its oxidation inside the intima, with the formation of the "foam cells." Toll-like receptors and inflamassomes participate in atherosclerosis formation, as well as in the activation of the complement system. In addition to innate immunity, adaptive immunity is also associated with atherosclerosis through antigen-presenting cells, T and B lymphocytes. AMI also increases the expression of some ILs and promotes macrophage and lymphocyte accumulation. Reperfusion increases the expression of anti-inflammatory ILs (such as IL-10) and generates oxygen free radicals. Although CAD and AMI are inflammatory disorders, the only drugs with anti-inflammatory effect so far widely used in ischemic heart disease are aspirin and statins. Some immunomodulatory or immunosuppressive promising therapies, such as cyclosporine and colchicine, may have benefits in CAD. Methotrexate also has potential cardioprotective anti-inflammatory effects, through increased adenosine levels. The TETHYS trial (The Effects of mETHotrexate Therapy on ST Segment Elevation MYocardial InfarctionS trial) will evaluate low-dose methotrexate in ST elevation AMI. The CIRT (Cardiovascular Inflammation Reduction Trial), in turn, will evaluate low-dose methotrexate in patients with a high prevalence of subclinical vascular inflammation. The CANTOS (The Canakinumab Antiinflammatory Thrombosis Outcomes Study) will evaluate canakinumab in patients with CAD and persistently elevated CRP. The blockage of other potential targets, such as the IL-6 receptor, CC2 chemokine receptor and CD20, could bring benefits in CAD.

B cells and humoral immunity in atherosclerosis

Insights into the important contribution of inflammation and immune functions in the development and progression of atherosclerosis have greatly improved our understanding of this disease. Although the role of T cells has been extensively studied for decades, only recently has the role of B cells gained more attention. Recent studies have identified differential effects of different B-cell subsets and helped to clarify the still poorly understood mechanisms by which these act. B1 cells have been shown to prevent lesion formation, whereas B2 cells have been suggested to promote it. Natural IgM antibodies, mainly derived from B1 cells, have been shown to mediate atheroprotective effects, but the functional role of other immunoglobulin classes, particularly IgG, still remains elusive. In this review, we will focus on recent insights on the role of B cells and various immunoglobulin classes and how these may mediate their effects in atherosclerotic lesion formation. Moreover, we will highlight potential therapeutic approaches focusing on B-cell depletion that could be used to translate experimental evidence to human disease.

Complement c3 is inversely associated with habitual intake of provitamin A but not with dietary fat, fatty acids, or vitamin E in middle-aged to older white adults and positively associated with intake of retinol in middle-aged to older white women

Complement factor 3 (C3) has been identified as a novel risk factor for obesity-associated cardiometabolic diseases. Data in the literature suggest that C3 concentrations may be influenced by diet. Therefore, we investigated the associations of intake of total fat, specific fatty acids, and fat-soluble vitamin E (and individual tocopherols) and vitamin A (and its dietary precursors) with circulating C3. In a white cohort [Cohort on Diabetes and Atherosclerosis Maastricht (CODAM); n = 501; 59.4 ± 7.1 y; 61% men], associations of habitual nutrient intake (assessed by a food-frequency questionnaire) with circulating C3 were evaluated by using cross-sectional multiple linear regression analyses. Adjustments were first performed for age, sex, glucose metabolism status (i.e., impaired glucose metabolism or type 2 diabetes), and energy intake and subsequently for BMI, waist circumference, alcohol intake, smoking behavior, and season of blood collection. No associations with C3 were observed for total dietary fat intake or intake of specific fatty acids [saturated, monounsaturated, polyunsaturated, n-6 (ω-6), and n-3 (ω- 3) fatty acids], vitamin E, or individual tocopherols. We observed an inverse association with intake of provitamin A carotenoids α-carotene (in μg/d; regression coefficient β = -0.075; 95% CI: -0.140, -0.010; P = 0.025) and β-carotene (in μg/d; β = -0.021; 95% CI: -0.044, 0.002; P = 0.068) with C3 (in mg/L). In contrast, and only in women, dietary retinol intake (in μg/d) was positively associated with C3 (β = 0.116; 95% CI: 0.014, 0.218; P = 0.026; n = 196). In conclusion, these data suggest that fasting concentrations of C3 may, in a complex manner, be modifiable by variation in dietary provitamin A carotenoids and/or retinol content of the usual diet but most likely not by variations in fat composition and vitamin E content.

Complement-mediated chronic inflammation is associated with diabetic microvascular complication

BACKGROUND

Chronic inflammation is characteristic of type 2 diabetes mellitus (T2DM). Obesity-activated adipocytes release adipocytokines, which induce the secretion of proinflammatory cytokines, resulting in vascular endothelial dysfunction and organ injury. C3a is a candidate to induce tissue inflammation.

METHODS

We investigated the association between diabetic microangiopathy and complement-mediated inflammation in 32 obese T2DM patients and 32 normal donors. Plasma levels of complement components and their activation intermediates were examined and related to the level of complication. An incubation study of post-prandial serum was carried out to measure the in vitro production of acylation stimulating protein (ASP/C3a desArg) by chylomicron.

RESULTS

Plasma levels of C3, C4, factor B, iC3b, Bb, and ASP were significantly increased in T2DM patients. Levels of C4d and membrane attack complex (C5b-9) were not significantly elevated. The activation rate of these factors indicated that only the early phase of alternative complement pathway was excessively activated. A statistical study revealed close correlation between ASP, body mass index, and highly sensitive C-reactive protein. Plasma ASP was significantly increased in the macroalbuminuric and proliferative retinopathy patient groups. An incubation study revealed that ASP was produced after the in vitro incubation of post-prandial serum from a T2DM patient with hyperchylomicronaemia.

CONCLUSIONS

Activation of the alternative complement pathway occurs in obese T2DM patients and is enhanced in the post-prandial hyperchylomicronic condition, which induces overproduction of ASP and C3a-mediated tissue inflammation. Therefore, complement-mediated inflammation may contribute to the acceleration of diabetic microangiopathy in addition to the development of macroangiopathy.

Elevated C1rC1sC1inh levels independently predict atherosclerotic coronary heart disease

Clinical studies as well as animal models emphasized the importance of the complement system in the pathogenesis of coronary atherosclerosis and cardiovascular diseases. Our aim was to examine the extent and clinical implication of complement system activation in patients with stable atherosclerotic coronary heart disease (ACHD). Seventy-six patients with stable angina pectoris (SAP) scheduled for elective coronary angiography were enrolled into the study. Percutaneous coronary intervention (PCI) was performed in 24 patients, in 27 patients (NOPCI group) the coronary angiography showed significant stenosis and bypass surgery (CABG) or optimal medical therapy (OMT) were advised, whereas in 25 patients the coronary angiography was negative (NC group). 115 volunteers served as healthy controls (HC). In all individuals, the plasma level of several complement activation products - C1rC1sC1inh, C3bBbP and SC5b-9 - were determined on admission, strictly before the coronary angiography. In patients with angiographically proven ACHD (PCI and NOPCI groups), the baseline C1rC1sC1inh levels were significantly higher compared to NC group and HC (p<0.0001, for both comparisons). According to the multiple logistic regression analysis, high C1rC1sC1inh level proved to be an independent biomarker of coronary heart disease (p<0.026, OR: 65.3, CI: 1.628-2616.284).

CONCLUSION

Activation of the classical complement pathway can be observed in angiographically proven coronary atherosclerosis. Elevated C1rC1sC1inh levels might represent an useful biomarker for coronary artery disease.

Complement and atherosclerosis-united to the point of no return?

Atherosclerosis is widely regarded as a chronic inflammatory disease that develops as a consequence of entrapment of oxidized low-density lipoprotein (LDL) in the arterial intima and its interaction with components of both innate and adaptive immunity. This article reviews the role of the complement system in the context of a different concept on atherogenesis. Arguments are forwarded in support of the contention that enzymatic and not oxidative modification of LDL is the prerequisite for transforming the lipoprotein into a moiety that is recognized by the innate immune system. In a departure from general wisdom, it is proposed that these processes are initially not pathological. To the contrary, they are physiological and meaningful because only thus can the stranded lipoprotein with its insoluble cargo, cholesterol, be removed from tissues. It is contended that histopathologically defined initial foam cell formation develops without inflammation and is reversible. Atherosclerosis as a disease evolves only when the cholesterol removal machinery is overloaded and it then represents a special type of immunopathological process primarily involving immune effectors of the innate rather than the adaptive immune system. This sets it apart from classical immunopathological reactions that are all based on dysfunctional adaptive immunity. But as with all other diseases of known origin, a defined molecular trigger, enzymatically modified-LDL (eLDL), exists whose intimal accumulation is required to initiate the pathologic process. And as with other diseases, the course of atherosclerosis will then be influenced by myriad genetic, endogenous, and environmental factors that by themselves, however, will not cause the disease. This simple concept is completely in line with general clinical experience and with the results of major clinical trials that have been conducted during the past decades.

Inhibition of inflammation and fibrosis by a complement C5a receptor antagonist in DOCA-salt hypertensive rats

The anaphylatoxin C5a generated by activation of the innate immunity complement system is a potent inflammatory peptide mediator through the G-protein-coupled receptor C5aR (CD88) present in immune-inflammatory cells, including monocytes, macrophages, neutrophils, T cells, and mast cells. Inflammatory cells infiltrate and initiate the development of fibrosis in the chronically hypertensive heart. In this study, we have investigated whether treatment with a selective C5aR antagonist prevents cardiovascular remodeling in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Control and DOCA-salt rats were treated with PMX53 (AcF-[OPdChaWR], 1 mg·kg·d oral gavage) for 32 days; structural and functional changes in cardiovascular system were determined. DOCA-salt hypertension increased leukocyte extravasation into ventricular tissue, increasing collagen deposition and ventricular stiffness; PMX53 treatment attenuated these changes, thereby improving cardiac function. Further, treatment with PMX53 suppressed an increased expression of C5aR in the left ventricle from DOCA-salt rats, consistent with the reduced infiltration of inflammatory cells. Vascular endothelial dysfunction in thoracic aortic rings was attenuated by PMX53 treatment, but systolic blood pressure was unchanged in DOCA-salt rats. In the heart, PMX53 treatment attenuated inflammatory cell infiltration, fibrosis, and ventricular stiffness, indicating that C5aR is critically involved in ventricular remodeling by regulating inflammatory responses in the hypertensive heart.

Membrane attack by complement: the assembly and biology of terminal complement complexes

Complement system activation plays an important role in both innate and acquired immunity. Activation of the complement and the subsequent formation of C5b-9 channels (the membrane attack complex) on the cell membranes lead to cell death. However, when the number of channels assembled on the surface of nucleated cells is limited, sublytic C5b-9 can induce cell cycle progression by activating signal transduction pathways and transcription factors and inhibiting apoptosis. This induction by C5b-9 is dependent upon the activation of the phosphatidylinositol 3-kinase/Akt/FOXO1 and ERK1 pathways in a Gi protein-dependent manner. C5b-9 induces sequential activation of CDK4 and CDK2, enabling the G1/S-phase transition and cellular proliferation. In addition, it induces RGC-32, a novel gene that plays a role in cell cycle activation by interacting with Akt and the cyclin B1-CDC2 complex. C5b-9 also inhibits apoptosis by inducing the phosphorylation of Bad and blocking the activation of FLIP, caspase-8, and Bid cleavage. Thus, sublytic C5b-9 plays an important role in cell activation, proliferation, and differentiation, thereby contributing to the maintenance of cell and tissue homeostasis.

Specific recognition of malondialdehyde and malondialdehyde acetaldehyde adducts on oxidized LDL and apoptotic cells by complement anaphylatoxin C3a

Oxidatively modified low-density lipoproteins (Ox-LDL) and complement anaphylatoxins C3a and C5a are colocalized in atherosclerotic lesions. Anaphylatoxin C3a also binds and breaks bacterial lipid membranes and phosphatidylcholine liposomes. The role of oxidized lipid adducts in C3a binding to Ox-LDL and apoptotic cells was investigated. Recombinant human C3a bound specifically to low-density lipoprotein and bovine serum albumin modified with malondialdehyde (MDA) and malondialdehyde acetaldehyde (MAA) in chemiluminescence immunoassays. No binding was observed to native proteins, LDL oxidized with copper ions (CuOx-LDL), or phosphocholine. C3a binding to MAA-LDL was inhibited by two monoclonal antibodies specific for MAA-LDL. On agarose gel electrophoresis, C3a comigrated with MDA-LDL and MAA-LDL, but not with native LDL or CuOx-LDL. C3a bound to apoptotic cells in flow cytometry. C3a opsonized MAA-LDL and was taken up by J774A.1 macrophages in immunofluorescence analysis. Complement-activated human serum samples (n=30) showed increased C3a binding to MAA-LDL (P<0.001) and MDA-LDL (P<0.001) compared to nonactivated samples. The amount of C3a bound to MAA-LDL was associated with total complement activity, C3a desArg concentration, and IgG antibody levels to MAA-LDL. Proteins containing MDA adducts or MAA adducts may bind C3a in vivo and contribute to inflammatory processes involving activation of the complement system in atherosclerosis.

Expression of the vitamin K-dependent proteins GAS6 and protein S and the TAM receptor tyrosine kinases in human atherosclerotic carotid plaques

The GAS6/ProS-TAM system is composed of two vitamin K-dependent ligands (GAS6 and protein S) and their three protein tyrosine kinase receptors TYRO3, AXL and MERTK, known as the TAM receptors. The system plays a prominent role in conditions of injury, inflammation and repair. In murine models of atherosclerotic plaque formation, mutations in its components affect atherosclerosis severity. Here we used Taqman low-density arrays and immunoblotting to study mRNA and protein expression of GAS6, ProS and the TAM receptors in human carotid arteries with different degrees of atherosclerosis. The results show a clear down-regulation of the expression of AXL in atheroma plaques with respect to normal carotids that is matched by decreased abundance of AXL in protein extracts detected by immunoblotting. A similar decrease was observed in PROS1 mRNA expression in atherosclerotic carotids compared to the normal ones, but in this case protein S (ProS) was clearly increased in protein extracts of carotid arteries with increasing grade of atherosclerosis, suggesting that ProS is carried into the plaque. MERTK was also increased in atherosclerotic carotid arteries with respect to the normal ones, suggesting that the ProS-MERTK axis is functional in advanced human atherosclerotic plaques. MERTK was expressed in macrophages, frequently in association with ProS, while ProS was abundant also in the necrotic core. Our data suggest that the ProS-MERTK ligand-receptor pair was active in advanced stages of atherosclerosis, while AXL signalling is probably down-regulated.

Complement in atherosclerosis: friend or foe?

Atherosclerosis is a chronic inflammatory disease and the complement system plays a central role in innate immunity. Increasing evidence exists that the complement system is activated within atherosclerotic plaques. However, the role of complement in atherogenesis is not fully understood. Whereas complement activation by the classic and lectin pathway may be protective by removing apoptotic cells and cell debris from atherosclerotic plaques, activation of the complement cascade by the alternative pathway and beyond the C3 convertase with formation of anaphylatoxins and the terminal complement complex may be proatherogenic and may play a role in plaque destabilization leading to its rupture and the onset of acute cardiovascular events. In this review article we present evidence for complement activation within atherosclerotic plaques and we discuss recent data derived from experimental animal models that suggest a dual role of complement in the development of the disease. In addition, we summarize the role of complement components as biomarkers for cardiovascular disease.

New insights of an old defense system: structure, function, and clinical relevance of the complement system

The complement system was discovered a century ago as a potent defense cascade of innate immunity. After its first description, continuous experimental and clinical research was performed, and three canonical pathways of activation were established. Upon activation by traumatic or surgical tissue damage, complement reveals beneficial functions of pathogen and danger defense by sensing and clearing injured cells. However, the latest research efforts have provided a more distinct insight into the complement system and its clinical subsequences. Complement has been shown to play a significant role in the pathogenesis of various inflammatory processes such as sepsis, multiorgan dysfunction, ischemia/reperfusion, cardiovascular diseases and many others. The three well-known activation pathways of the complement system have been challenged by newer findings that demonstrate direct production of central complement effectors (for example, C5a) by serine proteases of the coagulation cascade. In particular, thrombin is capable of producing C5a, which not only plays a decisive role on pathogens and infected/damaged tissues, but also acts systemically. In the case of uncontrolled complement activation, “friendly fire” is generated, resulting in the destruction of healthy host tissue. Therefore, the traditional research that focuses on a mainly positive-acting cascade has now shifted to the negative effects and how tissue damage originated by the activation of the complement can be contained. In a translational approach including structure-function relations of this ancient defense system, this review provides new insights of complement-mediated clinical relevant diseases and the development of complement modulation strategies and current research aspects.

The alternative pathway is critical for pathogenic complement activation in endotoxin- and diet-induced atherosclerosis in low-density lipoprotein receptor-deficient mice

BACKGROUND

The early components of the classical and lectin complement pathways have been shown to protect low-density lipoprotein receptor-deficient mice (Ldlr(-/-)) from early atherogenesis. However, the role of the alternative pathway remained unknown, and that was investigated in this study.

METHODS AND RESULTS

Mice lacking factor B (Bf(-/-)), the initiator of the alternative pathway, were crossed with Ldlr(-/-) mice and studied under different proatherogenic conditions. There was no statistically significant difference in lipid profiles or atherosclerotic lesion development between Bf(-/-)/Ldlr(-/-) and Ldlr(-/-) mice fed a low-fat diet. However, in these groups, administration of bacterial lipopolysaccharide led to a significant increase in atherosclerosis only in Ldlr(-/-) and not in Bf(-/-)/Ldlr(-/-) mice, indicating that the alternative pathway is necessary for endotoxin-mediated atherogenesis. Bf(-/-)/Ldlr(-/-) mice also had significantly decreased cross-sectional aortic root lesion fraction area and reduced lesion complexity compared with Ldlr(-/-) animals after a 12-week period of high-fat diet, although this was also accompanied by reduced levels of serum cholesterol. Under both experimental conditions, the atherosclerotic changes in the Bf(-/-)/Ldlr(-/-) mice were accompanied by a marked reduction in complement activation in the circulation and in atherosclerotic plaques, with no statistically significant differences in immunoglobulin G deposition or in the serum antibody response to oxidized low-density lipoprotein.

CONCLUSIONS

These data demonstrate that amplification of complement activation by the alternative pathway in response to lipopolysaccharide or high-fat diet plays a proatherogenic role.

Lack of complement inhibitors in the outer intracranial artery aneurysm wall associates with complement terminal pathway activation

Inflammation and activation of the complement system predispose to intracranial artery aneurysm (IA) rupture. Because disturbances in complement regulation may lead to increased susceptibility to complement activation and inflammation, we looked for evidence for dysregulation of the complement system in 26 unruptured and 26 ruptured IAs resected intraoperatively. Immunohistochemical and immunofluorescence results of parallel IA sections showed that deposition of the complement activation end-product C5b-9 was lacking from the luminal part of the IA wall that contained complement inhibitors factor H, C4b binding protein, and protectin as well as glycosaminoglycans. In contrast, the outer, less cellular part of the IA wall lacked protectin and had enabled full complement activation and C5b-9 formation. Decay accelerating factor and membrane cofactor protein had less evident roles in complement regulation. The Factor H Y402H variant, studied in 97 IA patients, was seen as often in aneurysm patients with or without aneurysm rupture as in the control population. The regulatory capacity of the complement system thus appears disturbed in the outer part of the IA wall, allowing full proinflammatory complement activation to occur before aneurysm rupture. Insufficient complement control might be due to matrix remodeling and cell loss by mechanical hemodynamics and/or inflammatory stress. Apparently, disturbed complement regulation leads to an increased susceptibility to complement activation, inflammation, and tissue damage in the IA wall.

C5a Receptor Targeting in Neointima Formation After Arterial Injury in Atherosclerosis-Prone Mice

Background— Receptor binding of complement C5a leads to proinflammatory activation of many cell types, but the role of receptor-mediated action during arterial remodeling after injury has not been studied. In the present study, we examined the contribution of the C5a receptor (C5aR) to neointima formation in apolipoprotein E–deficient mice employing a C5aR antagonist (C5aRA) and a C5aR-blocking monoclonal antibody.

Methods and Results— Mice fed an atherogenic diet were subjected to wire-induced endothelial denudation of the carotid artery and treated with C5aRA and anti-C5aR-blocking monoclonal antibody or vehicle control. Compared with controls, neointima formation was significantly reduced in mice receiving C5aRA or anti-C5aR-blocking monoclonal antibody for 1 week but not for 3 weeks, attributable to an increased content of vascular smooth muscle cells, whereas a marked decrease in monocyte and neutrophil content was associated with reduced vascular cell adhesion molecule-1. As assessed by immunohistochemistry, reverse transcription polymerase chain reaction, and flow cytometry, C5aR was expressed in lesional and cultured vascular smooth muscle cells, upregulated by injury or tumor necrosis factor-α, and reduced by C5aRA. Plasma levels and neointimal plasminogen activator inhibitor-1 peaked 1 week after injury and were downregulated in C5aRA-treated mice. In vitro, C5a induced plasminogen activator inhibitor-1 expression in endothelial cells and vascular smooth muscle cells in a C5aRA-dependent manner, possibly accounting for higher vascular smooth muscle cell immigration.

Conclusions— One-week treatment with C5aRA or anti-C5aR-blocking monoclonal antibody limited neointimal hyperplasia and inflammatory cell content and was associated with reduced vascular cell adhesion molecule-1 expression. However, treatment for 3 weeks failed to reduce but rather stabilized plaques, likely by reducing vascular plasminogen activator inhibitor-1 and increasing vascular smooth muscle cell migration.

The Role of Immune Complexes in Atherogenesis

Atherosclerosis is now recognized as a chronic inflammatory disease and is characterized by features of inflammation at all stages of its development. It also appears to display elements of autoimmunity, and several autoantibodies including those directed against oxidized low-density lipoprotein (ox-LDL) and heat shock proteins (Hsps) have been identified in atherosclerosis. Immune complexes (ICs) may form between these antigens and autoantibodies and via Fc receptor signaling and complement activation may modulate the inflammation in atherosclerosis. Antibody isotype may direct the role that ICs play in atherogenesis, immunoglobulin G (IgG) being potentially pro-atherogenic and immunoglobulin M (IgM) playing a protective role. Therapeutic options targeting complement activation and those which are potentially Fc-receptor mediated have been investigated in animal models, though targeting Fc receptor signaling is an area that needs further investigation.

C-reactive protein-mediated vascular injury requires complement

BACKGROUND

We previously demonstrated that vascular injury-induced neointima formation is exaggerated in human C-reactive protein (CRP) transgenic (CRPtg) compared to nontransgenic (NTG) mice. We now test the hypothesis that complement is required for this effect.

METHODS AND RESULTS

CRPtg and NTG with a normal complement system versus their counterparts lacking expression of complement component 3 (C3) protein (CRPtg/C3(-/-) and NTG/C3(-/-)) underwent carotid artery ligation. Twenty-eight days later, the injured vessels in CRPtg had thicker neointimas and more immunoreactive C3 in the surrounding adventitia compared with NTG. In CRPtg/C3(-/-), there was no increase in neointimal thickness compared with NTG or NTG/C3(-/-). Decreasing human CRP blood levels through administration of a selective antisense oligonucleotide eliminated the depletion of serum C3 associated with vascular injury and reduced immunoreactive C3 in the resultant lesions. In injured vessels, C3 colocalized with F4/80 (macrophage marker), and in vitro, human CRP elicited increased expression of C3 by bone marrow-derived macrophages.

CONCLUSIONS

Human CRP exaggeration of neointima formation in injured mouse carotid arteries associates with decreased circulating C3 and increased tissue-localized C3. C3 elimination or pharmacological reduction of human CRP prevents CRP-driven exacerbation of the injury response. In the CRPtg model system, mouse C3 is essential for the effect of human CRP.

Genetic control of the alternative pathway of complement in humans and age-related macular degeneration

Activation of the alternative pathway of complement is implicated in common neurodegenerative diseases including age-related macular degeneration (AMD). We explored the impact of common variation in genes encoding proteins of the alternative pathway on complement activation in human blood and in AMD. Genetic variation across the genes encoding complement factor H (CFH), factor B (CFB) and component 3 (C3) was determined. The influence of common haplotypes defining transcriptional and translational units on complement activation in blood was determined in a quantitative genomic association study. Individual haplotypes in CFH and CFB were associated with distinct and novel effects on plasma levels of precursors, regulators and activation products of the alternative pathway of complement in human blood. Further, genetic variation in CFH thought to influence cell surface regulation of complement did not alter plasma complement levels in human blood. Plasma markers of chronic activation (split-products Ba and C3d) and an activating enzyme (factor D) were elevated in AMD subjects. Most of the elevation in AMD was accounted for by the genetic variation controlling complement activation in human blood. Activation of the alternative pathway of complement in blood is under genetic control and increases with age. The genetic variation associated with increased activation of complement in human blood also increased the risk of AMD. Our data are consistent with a disease model in which genetic variation in the complement system increases the risk of AMD by a combination of systemic complement activation and abnormal regulation of complement activation in local tissues.

Complement system becomes activated by the classical pathway in intracranial aneurysm walls

Inflammation and activation of the complement system in the intracranial aneurysm (IA) wall predispose to IA rupture. We have previously shown that increased C5b-9 accumulation correlates with IA rupture and wall degeneration. To elucidate the underlying mechanisms, we investigated initiators and the pathway of complement activation in unruptured and ruptured IAs. Unruptured and ruptured IA wall samples were studied in parallel sections by immunohistochemical and immunofluorescence stainings for the location and relations of classical and alternative pathway complement components (C1q, C3b/iC3b, C3d, C4b/iC4b; n=35 and properdin, n=10), putative complement activators IgG (n=90), IgM, CRP and OxLDL (n=10), and complement activation endproduct C5b-9. Classical pathway components were seen in all IAs, and they were located mostly in the extracellular matrix. The early pathway complement components colocalized with each other, but were present in larger areas than C5b-9. The areas positive for complement component accumulation were significantly broader in ruptured than in unruptured IAs. The potential complement activators IgG, IgM, CRP and OxLDL were found mostly in the extracellular matrix and in partial overlap with C5b-9. Lipids were seen in Oil-Red-O staining in colocalization with C5b-9. Complement becomes activated by the classical pathway in the IA wall. The activation appears to be induced by multiple factors, which, in addition to the traditional activators (immunoglobulins, CRP, OxLDL), could involve vascular pressure-induced tissue damage. Despite wide early pathway activation, the terminal pathway is focused on a distinct lipid-rich layer. The profile of the complement components and the association of C5b-9 with lipids in the extracellular matrix indicate a long-term chronic inflammatory process rather than an acute targeted inflammatory reaction. The observed pattern of complement activation may be the consequence of local stress-induced insufficiency of complement regulation in IA walls.

Membrane binding and anticoagulant properties of protein S natural variants

INTRODUCTION

Protein S (PS) is a vitamin K-dependent plasma glycoprotein with a key role in the control of coagulation pathway on phospholipid membranes. We compared anticoagulant and membrane binding properties of PS altered by natural mutations (N217S, DelI203D204) affecting the epidermal growth factor like-domain 4 (EGF4) and causing PS deficiency.

MATERIALS AND METHODS

Binding of recombinant, immunopurified PS (rPS) to several conformation-specific antibodies, to C4BP and to phospholipid liposomes was investigated by ELISA. PS binding to cells was analysed by flow cytometry. PS inhibitory activities were studied in plasma and purified systems.

RESULTS AND CONCLUSIONS

Conformational changes produced by mutations were revealed by mapping with calcium-dependent antibodies. The immunopurified recombinant mutants (rPS) showed at 200-800 nM concentration reduced inhibition of coagulation (rPS217S, 10.2-17.3%; rPSDelI203D204, 5.8-8.9% of rPSwt) in FXa 1-stage clotting assay with APC. In thrombin generation assays the inhibition of ETP was reduced to 51.6% (rPS217S) and 24.1% (rPSDelI203D204) of rPSwt. A slightly shortened lag time (minutes) was also observed (rPS217S, 2.58; rPSDelI203D204, 2.33; rPSwt, 3.17; PS deficient plasma, 2.17). In flow cytometry analysis both mutants efficiently bound apoptotic cells in adhesion or in suspension. The affinity for phosphatidylserine-rich vesicles (apparent Kd: rPSwt 27.7+/-1.6 nM, rPS217S 146.0+/-16.1 nM and rPSDelI203D204 234.1+/-28.1 nM) was substantially increased by membrane oxidation (10.9+/-0.6, 38.2+/-3.5 and 81.4+/-6.0 nM), which resulted in a virtually normal binding capacity of mutants at physiological PS concentration. These properties help to define the molecular bases of PS deficiency, and provide further elements for PS-mediated bridging of coagulation and inflammation.

Human protein S inhibits the uptake of AcLDL and expression of SR-A through Mer receptor tyrosine kinase in human macrophages

Human protein S is an anticoagulation protein. However, it is unknown whether protein S could regulate the expression and function of macrophage scavenger receptor A (SR-A) in macrophages. Human THP-1 monocytes and peripheral blood monocytes were differentiated into macrophages and then treated with physiological concentrations of human protein S. We found that protein S significantly reduced acetylated low-density lipoprotein (AcLDL) uptake and binding by macrophages and decreased the intracellular cholesteryl ester content. Protein S suppressed the expression of the SR-A at both mRNA and protein levels. Protein S reduced the SR-A promoter activity primarily through inhibition in the binding of transcription factors to the AP-1 promoter element in macrophages. Furthermore, human protein S could bind and induce phosphorylation of Mer receptor tyrosine kinase (Mer RTK). Soluble Mer protein or tyrosine kinase inhibitor herbimycin A effectively blocked the effects of protein S on AcLDL uptake. Immunohistochemical analysis revealed that the level of protein S was substantially increased in human atherosclerotic arteries. Thus, human protein S can inhibit the expression and activity of SR-A through Mer RTK in macrophages, suggesting that human protein S is a modulator for macrophage functions in uptaking of modified lipoproteins.

The complement factor properdin induces formation of platelet-leukocyte aggregates via leukocyte activation

Both the complement system and platelet-leukocyte aggregates are involved in chronic and acute stages of atherosclerosis. Properdin, a positive regulator of the complement system, is secreted by leukocytes and endothelial cells. In the present study, the role of properdin in the formation of platelet-leukocyte aggregates was investigated. Incubation of human whole blood with properdin (25-200 microg/ml) resulted in a dose-dependent formation of platelet-leukocyte aggregates, with an increase of up to 2.2-fold compared to controls (p < 0.05), as analysed by flow cytometry. In addition, properdin significantly amplified ADP-induced aggregation of platelets with leukocytes by 53% (p < 0.05), while it had no effect on ADP-induced aggregation of platelets alone. Consistent with these results, properdin did not activate platelets as shown by the expression of activated GPIIb/IIIa (PAC-1 epitope) and P-selectin (CD62P) on the platelet surface. However, properdin significantly induced expression of CD11b (MAC-1) on leukocytes by 12-fold (p < 0.05) as a measure of leukocyte activation. In conclusion, the complement system component properdin induces the formation of platelet-leukocyte aggregates via leukocyte activation. The data establish a link between the complement system and platelet-leukocyte aggregates with potential significance in atherosclerotic vascular disease.

C4b-binding protein is present in affected areas of myocardial infarction during the acute inflammatory phase and covers a larger area than C3

BACKGROUND

During myocardial infarction reduced blood flow in the heart muscle results in cell death. These dying/dead cells have been reported to bind several plasma proteins such as IgM and C-reactive protein (CRP). In the present study we investigated whether fluid-phase complement inhibitor C4b-binding protein (C4BP) would also bind to the infarcted heart tissue.

METHODS AND FINDINGS

Initial studies using immunohistochemistry on tissue arrays for several cardiovascular disorders indicated that C4BP can be found in heart tissue in several cardiac diseases but that it is most abundantly found in acute myocardial infarction (AMI). This condition was studied in more detail by analyzing the time window and extent of C4BP positivity. The binding of C4BP correlates to the same locations as C3b, a marker known to correlate to the patterns of IgM and CRP staining. Based on criteria that describe the time after infarction we were able to pinpoint that C4BP binding is a relatively early marker of tissue damage in myocardial infarction with a peak of binding between 12 hours and 5 days subsequent to AMI, the phase in which infiltration of neutrophilic granulocytes in the heart is the most extensive.

CONCLUSIONS

C4BP, an important fluid-phase inhibitor of the classical and lectin pathway of complement activation binds to jeopardized cardiomyocytes early after AMI and co-localizes to other well known markers such as C3b.

Anti-inflammatory drugs and atherosclerosis

PURPOSE OF REVIEW

Inflammation contributes to the formation and progression of atherosclerosis and the therapeutic potential of some anti-inflammatory drugs has been evaluated for possible antiatherosclerotic effects. This review will briefly describe the mechanisms underlying the inflammation-atherosclerosis connection, the effect of various anti-inflammatory therapies on atherosclerotic disease and a sampling of the potential targets and agents under evaluation.

RECENT FINDINGS

Some agents with anti-inflammatory properties appear to have beneficial effects on atherosclerosis or subsequent risk for cardiovascular events, while others have been disappointing. The anti-inflammatory actions of statins have been linked retrospectively with their favorable effects on atherosclerosis progression and clinical outcomes. The cardiovascular safety of COX-2 inhibitors is being assessed prospectively in patients with atherosclerosis. Potential new therapeutic agents targeting other inflammatory mechanisms and oxidative stress are being evaluated in animal models and clinical trials.

SUMMARY

Due to the contributory inflammatory pathways in atherosclerosis, the properties of existing and novel anti-inflammatory agents are being carefully and actively evaluated in cardiovascular disease. Advances in our understanding of both atherosclerosis and the inflammatory contributors may play an important role in future strategies to decrease the incidence of atherosclerotic cardiovascular disease.